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Abstract:

An ink-jet recording method, having the steps of: ejecting droplets of
inks from an orifice of a recording head onto a recording sheet in
response to recording signals; and heating the recording sheet to
70° C. or higher before the droplets of the ink land on the
recording sheet or at the time of landing thereof, thereby to form a
multi-order color image on the recording sheet, in which the inks are two
or more kinds of inks that thicken by heating and have different color
hues from each other, and the droplets of the inks are ejected in
sequence from the ink having the lowest viscosity at 70° C. among
the inks.

Claims:

1. An ink-jet recording method, comprising the steps of: ejecting
droplets of inks from an orifice of a recording head onto a recording
sheet in response to recording signals; and heating the recording sheet
to 70.degree. C. or higher before the droplets of the ink land on the
recording sheet or at the time of landing thereof, thereby to form a
multi-order color image on the recording sheet, wherein the inks are two
or more kinds of inks that thicken by heating and have different color
hues from each other, and wherein the droplets of the inks are ejected in
sequence from the ink having the lowest viscosity at 70.degree. C. among
the inks.

2. The ink jet recording method according to claim 1, wherein the ink
comprises a pigment, water, and a heat-sensitive material.

3. The ink jet recording method according to claim 2, wherein the
above-described heat-sensitive material is a block polymer containing an
ethylene oxide moiety and a propylene oxide moiety.

4. The ink jet recording method according to claim 1, wherein each of the
two or more kinds of inks has a viscosity of 10 mPas or less at
25.degree. C. and 100 mPas or more at 70.degree. C.

5. The ink jet recording method according to claim 1, wherein the
recording sheet is printed in a single pass mode.

6. The ink-jet recording method according to claim 1, wherein the two or
more kinds of inks having different color hues comprise a yellow ink, a
magenta ink, a cyan ink, and a black ink.

9. The ink set according to claim 8, wherein each of the inks has a
viscosity of 10 mPas or less at 25.degree. C. and 100 mPas or more at
70.degree. C.

Description:

FIELD OF THE INVENTION

[0001] The present invention relates to an ink-jet recording method and an
ink set using the same.

BACKGROUND OF THE INVENTION

[0002] The ink-jet recording has such a lot of advantages that a
high-speed recording is possible; a low noise level is achieved;
colorization is easy; a high resolution can be made; and a plain paper
recording is possible. Because of these advantages, instruments and
equipments using such recording method are remarkably in widespread use.
As the ink used in this recording method, an aqueous ink is dominant from
the viewpoints of safety, odor and the like. In the ink-jet recording
method, image formation is performed by ejecting (discharging) the ink in
a rate of several thousands or more drops per second.

[0003] In the case where a high-speed printing is performed by the ink-jet
recording method, aggregation and color bleeding may occur. Specifically,
the term "aggregation" signifies a phenomenon in which before absorption
of the first ink droplet into a paper has been completed, the second ink
droplet reaches to the first ink droplet and they are united or
aggregated to form one large liquid droplet. The image resolution is
deteriorated by the aggregation. On the other hand, the term "color
bleeding" signifies a phenomenon in which image sharpness and color
quality are deteriorated on the grounds that two droplets, which are to
be united, contain a colorant having a different color from each other.

[0004] As a method contemplated to address the problem of color bleeding
in a high-speed printing, a method of using an ink that turns into a gel
in response to heat, and printing the ink on a recording element (paper)
having been heated at a higher temperature than that of the ink is
proposed (see JP-A-2003-285532 ("JP-A" means unexamined published
Japanese patent application)). Further, in order to resolve bleeding and
color bleeding as well as to form a highly coloring image, an ink-jet
aqueous ink containing a thermoreversible thickening polymer is proposed
(see JP-A-9-39381).

[0005] In the ink set used for a color print, inks based on four color
hues of yellow (Y), magenta (M), cyan (C) and black (B) are ordinarily
used in combination. In the time of image formation, inks are
sequentially ejected in each color and thereby printing is performed. In
the case where techniques disclosed in JP-A-2003-285532 and JP-A-9-39381
are used in the ink-jet recording that forms the above-described
multi-order color images, a high-temperature gelling ability of the ink
in each color hue depends on each of the ink composition. The present
inventors confirmed that in the multi-order color image formation,
bleeding of the dots at an overlap portion of the first-order color and
the second-order color may not be sufficiently prevented due to a
difference in the high-temperature gelling ability between the inks
having different color hue.

[0007] ejecting droplets of inks from an orifice of a recording head onto
a recording sheet in response to recording signals; and

[0008] heating the recording sheet to 70° C. or higher before the
droplets of the ink land on the recording sheet or at the time of landing
thereof, thereby to form a multi-order color image on the recording
sheet,

[0009] wherein the inks are two or more kinds of inks that thicken by
heating and have different color hues from each other, and

[0010] wherein the droplets of the inks are ejected in sequence from the
ink having the lowest viscosity at 70° C. among the inks.

[0011] Further, the present invention resides in an ink set, comprising
inks which thicken by heating,

[0013] wherein the inks satisfy the following relation in terms of
viscosity at 70° C. thereof:

[0014] the black ink<the cyan ink<the magenta ink<the yellow ink.

[0015] Other and further features and advantages of the invention will
appear more fully from the following description.

DETAILED DESCRIPTION OF THE INVENTION

[0016] According to the present invention, there are provided the
following means:

(1) An ink-jet recording method, comprising the steps of:

[0017] ejecting droplets of inks from an orifice of a recording head onto
a recording sheet in response to recording signals; and

[0018] heating the recording sheet to 70° C. or higher before the
droplets of the ink land on the recording sheet or at the time of landing
thereof, thereby to form a multi-order color image on the recording
sheet,

[0019] wherein the inks are two or more kinds of inks that thicken by
heating and have different color hues from each other, and

[0020] wherein the droplets of the inks are ejected in sequence from the
ink having the lowest viscosity at 70° C. among the inks.

(2) The ink-jet recording method described in the above item (1), wherein
the ink comprises a pigment, water, and a heat-sensitive material. (3)
The ink-jet recording method described in the above item (2), wherein the
above-described heat-sensitive material is a block polymer containing an
ethylene oxide moiety and a propylene oxide moiety. (4) The ink-jet
recording method described in any one of the above items (1) to (3),
wherein each of the two or more kinds of inks has a viscosity of 10 mPas
or less at 25° C. and 100 mPas or more at 70° C. (5) The
ink jet recording method described in any one of the above items (1) to
(4), wherein the recording sheet is printed in a single pass mode. (6)
The ink-jet recording method described in any one of the above items (1)
to (5), wherein the two or more kinds of inks having different color hues
comprise a yellow ink, a magenta ink, a cyan ink, and a black ink. (7)
The ink-jet recording method described in the above item (6), wherein the
inks satisfy the following relation in terms of viscosity at 70°
C. thereof:

[0023] wherein the inks satisfy the following relation in terms of
viscosity at 70° C. thereof:

[0024] the black ink<the cyan ink<the magenta ink<the yellow ink.

(9) The ink set described in the above item (8), wherein each of the inks
has a viscosity of 10 mPas or less at 25° C. and 100 mPas or more
at 70° C.

[0025] First, the ink-jet recording method of the present invention will
be described.

[Ink-Jet Recording Method]

[0026] The ink jet recording method is a method to form images by ejecting
ink droplets from a plurality of nozzles or orifices built in a recording
head of an ink-jet printer, and allowing the ink droplets to land on a
recording sheet while controlling the ink droplets by the ejection. This
method is roughly classified into a method of ejecting liquid droplets by
applying a mechanical energy to the liquid droplets and a method of
ejecting liquid droplets by bubble release resulting from application of
heat energy to the liquid droplets. In the present invention, any one of
these methods may be used. A piezo-type recording head is preferably
used. If a thermal type head is used, an ink may thicken due to heat at
the time of ejecting the ink. As a result, the ejection direction or
ejection amount of the ink may become unstable. In some cases, ejection
may become impossible. In contrast, use of the piezo head makes it
possible to eject the ink very well, so the piezo head is preferable.

[0027] As the method of recording a color image, there are a shuttle scan
mode and a single pass mode. In the shuttle scan mode (shuttle scan
system), recording is performed by scanning a head relative to the same
recording portion more than once. On the other hand, in the single pass
mode (single pass system), recording is performed by scanning only once a
head relative to a recording portion. Of these modes, the recording
method of the present invention is favorably suitable for the single pass
mode. The recording method of the present invention has the following
characteristics due to use of a high-temperature gelling ink described
later. After landing of the ink onto a recording sheet, hard dots are
formed in a short time. As a result, in the single pass mode in which the
second-order color lands directly after landing of the first-order color,
both color bleeding and dot bleeding are prevented. Accordingly, effects
such as enhancement of image quality are achieved.

[0028] The printing speed is not particularly limited. However, since the
present invention makes it possible to obtain excellent images even in a
high-speed printing, the range of from 50 m/min to 200 m/min is
preferred. Though the liquid amount per droplet is not particularly
limited, the range of from 2 to 15 pl is preferred.

[0029] In the ink-jet recording method of the present invention, image
formation is performed by ejecting ink droplets from a recording head. In
addition, the ink jet recording method of the present invention is
characterized in that the recording sheet is heated to 70° C. or
higher before the ink droplets land, or at the time of landing,
preferably in the range of from 70° C. to 100° C., more
preferably in the range of from 70° C. to 90° C., and still
more preferably in the range of from 70° C. to 80° C. This
temperature is defined as a value obtained by measuring a temperature at
the side of the ink jet recording (the side at which ink droplets land)
of the recording sheet using a noncontact thermometer such as an infrared
thermometer (for example, IR-66B (trade name), manufactured by MK
Scientific, Inc.). The measuring position is set between a head section
of the ink jet recording apparatus and a means (unit) that heats a
recording sheet. If the heating temperature is too low, the ink may not
sufficiently thicken. Further, in order to heat to a higher temperature
than the above-described temperature, extra heat sources are needed. As a
result, the extra heat sources become a load on the system. In addition,
heating may be carried out both before the ink droplets land and at the
time of landing. In the method of the present invention, the thickening
of the ink on a recording sheet is accelerated by heating on the
recording sheet whereby, for example, bleeding can be suppressed.

[0030] The ink jet recording method of the present invention is further
characterized in that the lower the viscosity of the ink at 70° C.
is, among the inks having color hues that are different from each other,
the earlier the ejecting order is. By adjusting a high-temperature
gelling ability of the ink according to the ejecting order, dot bleeding
at the overlap portion between dots having different colors from each
other can be prevented. This phenomenon appears to be caused by the
following reasons.

[0031] In the case where dots are formed with the second ink (second-order
ink) on an image having been formed with the first ink (first-order ink),
penetration of water into the ink is delayed, as compared with the case
where dots are directly formed at a non-image area. Accordingly, the
overlap portion between dots having different colors from each other is
inferior in efficiency of increasing viscosity that accompanies a
temperature rise of the ink due to heating, and resultantly bleeding of
the ink tends to become worse. In the case where the first ink is more
likely to thicken at a high temperature than the second ink, the second
ink on the formed image becomes more difficult to penetrate. As a result,
this tendency becomes more conspicuous. Accordingly, when the
high-temperature gelling ability of the ink that forms an image in first
is lower than that of the ink that forms an image on the
previously-formed image, dot bleeding at the overlap portion between
these images is hardly caused.

[0032] In the case where inks each having the same viscosity at 70°
C. are contained, ejecting may be carried out regardless of the inks.
Given the quality of the obtained image, however, it is preferred to use
the ink of lower saturation first and the ink of higher saturation later.

[0033] In the ink-jet recording method of the present invention, the inks
having different color hues from each other preferably contain inks of at
least yellow, magenta, cyan and black. Further, it is especially
preferred to use an ink set composed of inks that satisfy the following
relation of viscosity at 70° C.: black ink<cyan ink<magenta
ink<yellow ink, and in addition to print in the order of the black
ink, the cyan ink, the magenta ink and the yellow ink. The
above-described ejecting order is favorable since a sharp print quality
is obtained without causing bleeding at the overlap portion between the
different colors.

[0034] In the ink jet recording method of the present invention, two or
more kinds of inks having different color hues from each other are used.
For example, an embodiment where an ink of other color hue may be also
used together with the above-described four kinds of inks having
different color hues from each other is practiced without any particular
limitation. In order to achieve broader color reproduction range, for
example, the ink set may contain a part or all of special colors such as
red, green and blue. With respect to the ejecting order in this case, it
is preferred to eject in such an order that the ink having the lowest
viscosity at 70° C. is first as described above. It is more
preferred that the viscosity at 70° C. of each of the inks to be
used is adjusted so as to become higher in the order as the saturation
thereof becomes higher.

[0035] The recording sheet to be used may be, but not limited to, a sheet
of general printing paper (plain paper) containing cellulose as a main
component, such as so-called high-quality paper, coated paper, or art
paper. When general printing paper containing cellulose as a main
component is used in image recording by a conventional ink-jet method
with a water-based ink, the ink may be absorbed in the paper and dried
relatively slowly, so that colorants in the ink may be likely to migrate
after being provided on the paper, which may easily lead to image quality
deterioration. According to the ink-jet recording method of the
invention, however, the migration of the colorants (pigments) may be
suppressed so that high-quality image recording with good color density
and suppression of penetration of the ink may be achieved.

[0038] In the ink-jet recording method of the present invention, at least
two kinds of inks that thicken in response to heating are used. The
phrase "thickens in response to heating" signifies that the difference in
ink viscosity between at 25° C. and at 70° C. is 70 mPas or
more.

[0039] Hereinafter, the ink which is used in the present invention, whose
viscosity thickens in response to heating is referred to as "a
high-temperature gelling ink".

[0040] The high-temperature gelling ink of the present invention contains
a heat-sensitive material to be hereinafter described. The viscosity at
70° C. is preferably 100 mPas or more, and more preferably 150
mPas or more. The upper limit of the viscosity at 70° C. is not
particularly restricted. A relatively higher viscosity is preferred.
However, the viscosity is ordinarily 10,000 mPas or less.

[0041] Further, the viscosity at 25° C. used in the present
invention is preferably 10 mPas or less, and more preferably from 2 to 8
mPas. When the viscosity of the ink is in the above range, ejectability
from the ink-jet head can well be controlled.

[0042] In addition, a measuring method of the viscosity in the present
invention is as follows.

(Measuring Method of Viscosity)

[0043] Unless otherwise indicated, the viscosity in the present invention
refers to an average of the values obtained by measuring a viscosity five
times every 100 seconds after the test sample has been adjusted to a
predetermined temperature using a temperature-variable type rotational
viscometer Physica MCR301 (trade name, manufactured by Anton Paar GmbH).
It can be assumed that the viscosity obtained by the above measurement is
also achieved on a recording sheet having been heated according to the
recording method of the present invention described below. As the
measuring conditions, shear rate of 10 (1/s) and rate of temperature rise
of 5° C./5 seconds are used.

[0044] The thickening behavior of the high-temperature gelling ink is
assumed as follows. The heat-sensitive material in the ink is a polymer
that undergoes dissociative resolution and associative thickening at a
given transition temperature. When the polymer is dissolved in a medium
by hydration, the polymer dehydrates by heating. As a result, the polymer
molecules interact with each other whereby the ink turns into a gel,
resulting in thickening.

[0045] According to the ink of the present invention, it is possible to
suppress both aggregation and color bleeding of the ink droplets in a
high-speed printing because the ink of the present invention has the
above properties.

[0046] Further, in the case of forming dots with such
high-temperature-induced gelation ink according to the ink-jet recording
method, a solvent evaporates after increase of viscosity due to gel
transition. As a result, the cross-sectional shape of the dot forms a
trapezoid or concave. From the viewpoint of density uniformity, a
trapezoid is preferred. In the ink of the present invention, the shape of
the dots to be formed is excellent whereby print qualities are also
improved.

[Heat-Sensitive Material]

[0047] The high-temperature gelling ink contains a heat-sensitive material
that thickens by heating.

[0048] As one example of the heat-sensitive materials, a water-soluble
cellulose ether compound is exemplified. Examples of the water-soluble
cellulose ether compound used in the present invention include
hydroxypropyl cellulose, hydroxypropyl methylcellulose, methylcellulose,
and hydroxybutoxyl-modified methylcellulose/hydroxypropyl
methylcellulose. As commercially available products of these compounds,
for example, METHOCEL K100 LV, METHOCEL A-15C, or METHOCEL HB (all trade
names, manufactured by The Dow Chemical Company) can be favorably used.

[0049] As another example of the heat-sensitive materials, a polymer
having at least one polyethyleneoxide (PEO) block structure is
exemplified. Specific examples of the polymer include polyethylene oxide,
di-block polymer of polyethylene oxide-polypropylene oxide (PEO-PPO),
di-block polymer of polyethylene oxide-polycaprolactone, di-block polymer
of polyethylene oxide-polylactide, and tri-block copolymer of
polyethylene oxide-poly propylene oxide-polyethylene oxide (PEO-PPO-PEO).
A block polymer having a PEO moiety (unit) and a PPO moiety is preferred.
Further, it is especially preferred to use an aqueous solution of a
PEO-PPO-PEO tri-block copolymer. The copolymerization ratio is preferably
in the range of from 10 to 100% by mass, more preferably from 40 to 100%
by mass, and especially preferably from 60 to 90% by mass, in terms of
the percentage by mass of PEO contained in the above-described polymer.
The allocation of PEO allocated to both sides of the PEO block is not
particularly limited. However, the allocation is preferably in the range
of from 1:99 to 50:50, more preferably from 10:90 to 50:50, and still
more preferably from 20:80 to 50:50.

[0050] The molecular weight of the above polymer is preferably from 1,000
to 100,000, more preferably from 8,000 to 30,000.

[0051] When described simply as a molecular weight in the present
invention, the molecular weight means a number average molecular weight
unless otherwise specified, and the molecular weight is a value measured
by the following measuring method.

(Measuring Method of Molecular Weight)

[0052] The molecular weight is measured using GPC (gel permeation
chromatography) method, unless otherwise specified. The gel packed in the
column used for GPC method is preferably a gel having an aromatic
compound in the repeating unit, and examples thereof include a gel
comprising a styrene-divinylbenzene copolymer. Two to six columns are
preferably connected and used. The solvent used includes an ether-based
solvent such as tetrahydrofuran, and an amide-based solvent such as
N-methylpyrrolidinone, and an ether-based solvent such as tetrahydrofuran
is preferred. The measurement is preferably performed at a solvent flow
rate of 0.1 to 2 mL/min, most preferably from 0.5 to 1.5 mL/min. When the
measurement is performed in this range, the measurement can be performed
more efficiently without imposing a load on the apparatus. The
measurement temperature is preferably from 10 to 50° C., and most
preferably from 20 to 40° C.

[0053] The specific conditions for the measurement of molecular weight are
shown below.

[0069] The addition amount of the heat-sensitive material is not
particularly limited, as long as a thickening effect due to heating (the
viscosity of the ink at 70° C. satisfies the above predetermined
value) is sufficiently obtained and the tri-block copolymer achieves
viscosity such that the ink is ejected from a head of the recording
apparatus. However, the heat-sensitive material is added in the range of
preferably from 2% by mass to 20% by mass, and more preferably from 5% by
mass to 15% by mass. If the addition amount is too small, a thickening
effect sometimes may not be sufficiently obtained. On the other hand, if
the addition amount is too large, ink viscosity before heating becomes
too high, so that ejection of the ink from a head of the recording
apparatus sometimes may be affected.

[0070] Further, two or more kinds of heat-sensitive materials may be used
in combination in the present invention. In this case, it is preferred to
set a total content of the heat-sensitive material within the
above-described range.

[0071] Further, it is preferred that the heat-sensitive material used in
the present invention exists in the ink in a solution state.

[Pigment]

[0072] As the pigment in the present invention, any known pigment can be
used without any particular restriction. Above all, a pigment that is
substantially insoluble or sparingly soluble in water is preferred from
the standpoint of ink coloring properties. In the present invention, a
water-insoluble pigment itself or a pigment itself surface-treated with a
dispersant can be used as the pigment (colorant).

[0073] The pigment that may be used in the present invention is not
particularly limited in its kind, and any one of the conventional organic
and inorganic pigments may be used. Examples of the pigment that may be
used include polycyclic pigments such as azo lake, azo pigment,
phthalocyanine pigment, perylene and perynone pigments, anthraquinone
pigment, quinacridone pigment, dioxadine pigment, diketopyrrolopyrrole
pigment, thioindigo pigment, isoindolinone pigment and quinophthalone
pigment; dye lakes such as basic dye type lake and acidic dye type lake;
organic pigments such as nitro pigment, nitroso pigment, aniline black
and daylight fluorescent pigment; and inorganic pigments such as titanium
oxide, iron oxide type and carbon black type. Even pigments that are not
described in Color Index can be used so long as they are pigments capable
of being dispersed in an aqueous phase. Furthermore, those obtained by
surface-treating the above-described pigments with a surfactant, a
polymeric dispersant or the like, and grafted carbon can also be used. Of
the above pigments, azo pigment, phthalocyanine pigment, anthraquinone
pigment, quinacridone pigment and carbon black type pigment are
preferably used. For black pigments, it is especially preferred to use
carbon black type pigments.

[0074] Specific examples of the organic pigment used in the present
invention are described below.

[0079] The average particle diameter of the pigment is preferably from 10
to 200 nm, more preferably from 10 to 150 nm, and further preferably from
10 to 100 nm. When the average particle diameter is 200 nm or less,
favorable color reproducibility and dotting properties upon dotting by an
ink-jet method can be achieved. When the average diameter is 10 nm or
more, favorable light fastness can be achieved. The particle size
distribution of the pigment is not particularly limited, and the pigment
may have a wide range of particle size distribution or a monodispersible
particle size distribution. Further, two or more kinds of pigment each
having a monodispersible particle size distribution may be used in
combination.

[0080] The average particle diameter and the particle size distribution of
the pigment can be obtained by measuring the volume-average particle
diameter of the pigment by a dynamic light scattering method, using a
NANOTRACK particle size distribution analyzer (UPA-EX150, trade name,
manufactured by Nikkiso Co., Ltd.).

[0081] The pigment may be used alone or in combination of two or more
kinds. From the viewpoint of image density, the content of pigment in the
ink is preferably from 1% by mass to 25% by mass, more preferably from 2%
by mass to 20% by mass, still more preferably from 5% by mass to 20% by
mass, and particularly preferably from 5% by mass to 15% by mass, with
respect to the total amount of the ink composition.

[Dispersant and Dispersing Medium]

[0082] Ordinarily the dispersant is a material to be added for the purpose
of dispersing a pigment, and the dispersing medium (binder) is a material
to be added for the purpose of improving scratch resistance, solvent
resistance, water resistance, and the like. However, in the present
invention, a material that is described bellow as the dispersant may be
added as a dispersing medium. Accordingly, the dispersant and the
dispersing medium are collectively explained below as a dispersant.

[0083] The pigment according to the present invention is preferably
dispersed in an aqueous solvent by a dispersant. The dispersant may be a
polymer dispersant, or a surfactant type dispersant. The polymer
dispersant may be either one of a water-soluble dispersant or a
water-insoluble dispersant.

[0084] The above surfactant dispersant can be added for the purpose of
dispersing an organic pigment in an aqueous medium while maintaining the
viscosity of the ink at a low level. The surfactant dispersant referred
herein is a dispersant of which molecular weight is smaller than the
polymer dispersant, and the surfactant dispersant has a mass average
molecular weight of 2,000 or less. The molecular weight of the surfactant
dispersant is preferably from 100 to 2,000, and more preferably from 200
to 2,000.

[0088] Among those, a polymer compound containing a carboxyl group or a
sulfonyl group is preferable from the viewpoint of dispersion stability
of pigment. Polymer compounds containing a carboxyl group such as the
following are particularly preferable: (meth)acrylic resins such as
styrene-(meth)acrylic resins; styrene maleic acid resins;
vinylnaphthalene acrylic resins; vinylnaphthalene maleic acid resins,
polyvinylbenzenesulfonate resins, polystyrene-vinylbenzenesulfonate
resins, and styrene-vinylbenzenesulfonate resins.

[0089] The mass average molecular weight of the polymer dispersant in the
present invention is preferably from 3,000 to 200,000, more preferably
from 5,000 to 100,000, further preferably from 5,000 to 80,000, and yet
further preferably from 10,000 to 60,000.

[0090] The ratio of an amount of the pigment to an amount of the
dispersant (pigment:dispersant) in the ink composition in terms of mass
is preferably in a range of from 1:0.06 to 1:3, more preferably in a
range of from 1:0.125 to 1:2, and still more preferably in a range of
from 1:0.125 to 1:1.5.

[Solvent]

[0091] The ink of the present invention is an aqueous ink. As the solvent,
water, more preferably ion-exchanged water is used. Any other organic
solvent may be contained for the purpose of suppressing drying,
accelerating penetration, regulating viscosity, and the like.

[0092] A certain organic solvent used as an anti-drying agent can be
effectively prevent nozzle clogging, which could otherwise be caused by
the ink dried in the ink discharge port in the process of discharging the
ink composition by ink jet method for image recording.

[0093] For the suppression of drying, a water-soluble organic solvent
having a vapor pressure lower than that of water is preferably used.
Examples of the water-soluble organic solvent suitable for the
suppression of drying include: polyhydric alcohols such as ethylene
glycol, propylene glycol, diethylene glycol, polyethylene glycol,
thiodiglycol, dithiodiglycol, 2-methyl-1,3-propanediol,
1,2,6-hexanetriol, acetylene glycol derivatives, glycerin, and
trimethylolpropane; heterocyclic compounds such as 2-pyrrolidone,
N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, and
N-ethylmorpholine; sulfur-containing compounds such as sulfolane,
dimethylsulfoxide and 3-sulfolene; polyfunctional compounds such as
diacetone alcohol and diethanolamine; and urea derivatives. In
particular, polyhydric alcohols such as glycerin and diethylene glycol
are preferred.

[0094] In order to accelerate the penetration, an organic solvent may be
used for better penetration of the ink composition into recording media.
Examples of the organic solvent suitable for penetration acceleration
include alcohols such as ethanol, isopropanol, butanol, and
1,2-hexanediol, sodium lauryl sulfate, sodium oleate, and nonionic
surfactants.

[0095] Besides, a water-soluble organic solvent may also be used to
control viscosity. Examples of the water-soluble organic solvent that may
be used to control viscosity include alcohols (e.g., methanol, ethanol
and propanol), amines (e.g., ethanolamine, diethanolamine,
triethanolamine, ethylenediamine, and diethylenetriamine), and other
polar solvents (e.g., formamide, N,N-dimethylformamide,
N,N-dimethylacetamide, dimethylsulfoxide, sulfolane, 2-pyrrolidone,
acetonitrile, and acetone).

[0096] The content of the organic solvent is preferably from 0% by mass to
80% by mass, more preferably from 0% by mass to 60% by mass, and still
more preferably from 0% by mass to 50% by mass with respect to the total
amount of the ink.

[Water]

[0097] The ink composition used in the invention contains water. There is
no particular limitation to the content of water in the ink composition.
The content of water may be from 10% by mass to 99% by mass, more
preferably from 30%% by mass to 80% by mass, and still more preferably
50% by mass to 70% by mass with respect to the total amount of the ink
composition.

[Nitrogen-Containing Compound]

[0098] The ink used in the present invention may contain the compound
represented by the following formula (1-1) or formula (2-1) and may
contain at least two kinds of these compounds. In the ink jet recording
method of the present invention, the heating temperature of the recording
sheet can be suppressed to a lower temperature by this compound. In the
ink used in the present invention, when a heat-sensitive material is
dissolved in a medium by hydration, the heat-sensitive material
dehydrates by heating. As a result, the heat-sensitive materials interact
with each other. It is presumed that the ink turns into a gel by the
above interaction, resulting in thickening. On the other hand, it is
assumed that the compound represented by the formula (1-1) or formula
(1-2) has a hydrogen-bonding property whereby dehydration of the
heat-sensitive material is accelerated.

##STR00001##

[0099] wherein R11, R12, R13 and R14 each
independently represent a hydrogen atom or an alkyl group having 1 to 4
carbon atoms; R13 and R14 may be bonded to each other to form a
ring; and X represents an oxygen atom or a sulfur atom; and

##STR00002##

[0100] wherein R21 represents a hydrogen atom or an alkyl group
having 1 to 4 carbon atoms; Y represents an oxygen atom or a sulfur atom;
and n represents an integer of 1 to 3.

[0101] In formula (1-1), specific examples of R11, R12, R13
and R14 include a hydrogen atom, a methyl group, an ethyl group, a
propyl group and a butyl group. Each of R11, R12, R13 and
R14 is preferably a hydrogen atom, a methyl group, an ethyl group or
a propyl group; more preferably a hydrogen atom or a methyl group.

[0102] In formula (1-1), R13 and R14 may be bonded to each other
to form a ring. Specific examples of an alkylene group, which is formed
by bonding, include an ethylene group, a propylene group and a butylene
group. The alkylene group is preferably an ethylene group or a propylene
group.

[0103] In formula (2-1), Y is preferably an oxygen atom.

[0104] In formula (2-1), specific examples of R21 include a hydrogen
atom, a methyl group, an ethyl group, a propyl group and a butyl group.
R21 is preferably a hydrogen atom, a methyl group, an ethyl group or
a propyl group; more preferably a hydrogen atom or a methyl group.

[0105] In formula (2-1), n is preferably an integer of 1 to 2, and more
preferably 1.

[0106] Further, the compound represented by formula (1-1) or formula (2-1)
is preferably the compound represented by formula (1-2) or (2-2),
respectively.

##STR00003##

[0107] wherein R11, R12 and X have the same meanings as
R11, R12 and X in formula (1-1), respectively; and the specific
examples and favorable ranges thereof are also the same as those in
formula (1-1).

##STR00004##

[0108] wherein Y and n have the same meanings as Y and n in formula (2-1),
respectively, and the specific examples and favorable ranges thereof are
also the same as those in formula (2-1).

[0109] More preferable compound represented by formula (1-2) or formula
(2-2) is represented by the following formula (1-3) or formula (2-3)
respectively.

##STR00005##

[0110] wherein R11 and R12 have the same meanings as R11
and R12 in formula (1-1), respectively, and the specific examples
and favorable ranges thereof are also the same as those in formula (1-1).

##STR00006##

[0111] wherein n has the same meaning as n in formula (2-1), and the
specific examples and favorable range thereof are also the same as those
in formula (2-1).

[0113] Specific preferable examples of the compound represented by
formulae (2-1) to (2-3) include 2-pyrrolidone and N-methylpyrrolidone.
Among them, 2-pyrrolidone is more preferable.

[0114] As the addition amount of the nitrogen-containing compound, it is
preferred to add the nitrogen-containing compound to the ink in an amount
of more than 7% by mass, more preferably more than 7% by mass and 40% by
mass or less, and still more preferably more than 7% by mass and 25% by
mass or less. If the addition amount is too small, the thickening effect
due to addition of the nitrogen-containing compound may not be obtained
sufficiently. On the other hand, if the addition amount is too large, the
ink viscosity at room temperature increases, so that ejection may become
difficult.

[0115] In addition, the addition amount of the nitrogen-containing
compound is adjusted so that the thickening behavior of the ink becomes
as described above. Further, the addition amount of the
nitrogen-containing compound is preferably in the range of from 0.23 to
40% by mass in terms of the ratio by mass of the nitrogen-containing
compound with respect to the heat-sensitive material.

[Other Additives]

[0116] The ink composition of the invention may further contain other
components (additives) in accordance with necessity. Examples of such
other components include known additives such as an anti-fading, an
emulsion stabilizer, a permeation accelerator, an ultraviolet absorber, a
preservative, an antifungal agent, a pH regulator, a surface tension
regulator, a defoaming agent, a viscosity adjusting agent, a dispersant,
a dispersion stabilizer, an anti-rust agent or a chelating agent. Those
various additives may directly be added after preparation of the ink
composition, or may be added at the time of preparation of the ink
composition.

[Ultraviolet Absorber]

[0117] The ultraviolet absorber is used for the purpose of improving
preservability of an image. The ultraviolet absorber can use
benzotriazole compounds described in, for example, JP-A Nos. 58-185677,
61-190537, 2-782, 5-197075 and 9-34057; benzophenone compounds described
in, for example, JP-A Nos. 46-2784 and 5-194483, and U.S. Pat. No.
3,214,463; cinnamic acid compounds described in, for example, JP-B Nos.
48-30492 and 56-21141, and JP-A No. 10-88106; triazine compounds
described in, for example, JP-A Nos. 4-298503, 8-53427, 8-239368 and
10-182621, and JP-A No. 8-501291; compounds described in Research
Disclosure No. 24239; and compounds that absorb ultraviolet light and
emit fluorescence, i.e., fluorescent brighteners, represented by stilbene
compounds or benzoxazole compounds.

[Color Fading Inhibitor]

[0118] The color fading inhibitor is used for the purpose of improving
storability of an image. Examples of the color fading inhibitor that can
be used include various organic color fading inhibitors and metal complex
color fading inhibitors. Examples of the organic color fading inhibitor
include hydroquinones, alkoxyphenols, dialkoxyphenols, phenols, anilines,
amines, indanes, chromenes, alkoxyanilines and heterocycles. Examples of
the metal complex color fading inhibitor include a nickel complex and a
zinc complex. More specifically, compounds described in the patents cited
in Research Disclosure No. 17643, chapter VII, items Ito J; Research
Disclosure No. 15162: Research Disclosure No. 18716, page 650, the
left-hand column; Research Disclosure No. 36544, page 527; Research
Disclosure No. 307105, page 872; and Research Disclosure No. 15162, and
compounds included in the formulae of the representative compounds and
the exemplified compounds described on pages 127 to 137 of JP-A No.
62-215272 can be used.

[Mildew-Proofing Agent]

[0119] Examples of the mildew-proofing agent include sodium
dehydroacetate, sodium benzoate, sodium pyridinethion-1-oxide,
p-hydroxybenzoic acid ethyl ester, 1,2-benzisothiazolin-3-one and its
salt. Those are preferably used in the water-based ink composition in an
amount of from 0.02 to 1.00% by mass.

[pH Regulator]

[0120] As the pH regulator, a neutralizer (organic base and inorganic
alkali) may be used. The pH regulator may be added in an amount such that
the water-based ink composition has pH of preferably from 6 to 10, and
more preferably from 7 to 10, for the purpose of improving storage
stability of the water-based ink composition.

[0122] For smooth ejection in the ink-jet recording method, the amount of
addition of the surface tension regulator is preferably such that the
surface tension of the ink composition can be adjusted in the range of
from 20 mN/m to 60 mN/m, more preferably from 20 mN/m to 45 mN/m, further
preferably from 25 mN/m to 40 mN/m.

[0123] The surface tension of the ink composition may be measured by a
plate method using AUTOMATIC SURFACE TENSIOMETER CBVP-Z (trade name,
manufactured by Kyowa Interface Science Co., LTD.) under the temperature
condition of 25° C.

[0125] The surfactants listed in pages 37 to 38 of JP-A No. 59-157636 and
Research Disclosure No. 308119 (1989) may also be used.

[0126] Fluorocarbon (alkyl fluoride type) surfactants or silicone
surfactants as described in JP-A Nos. 2003-322926, 2004-325707 and
2004-309806 may be used to improve the rubbing resistance.

[0127] The surface tension regulator may also be used as a defoaming
agent, and fluorine compounds, silicone compounds, and chelating agents
such as EDTA may also be used.

[0128] In the ink of the present invention, a thickener, a conductivity
improver, a kogation inhibitor ("kogation" means solid deposits baked
onto the surface of a heater), a desiccant, a water-resistant
ruggedization agent, a light stabilizer, a buffering agent, an
anti-curling agent, or the like further may be added. Examples of the
buffering agent include sodium borate, sodium hydrogenphosphate, sodium
dihydrogenphosphate, and a mixture thereof. However, the buffering agent
is not limited thereto.

[0129] Next, the ink set of the present invention will be described.

[Ink Set]

[0130] The ink set of the present invention contains at least a yellow hue
ink, a magenta hue ink, a cyan hue ink, and a black hue ink. Further,
these inks are high-temperature gelling inks described above. Further, it
is preferred that these inks satisfy the following relation of viscosity
at 70° C.: black ink<cyan ink<magenta ink<yellow ink.
These ink set may further contain inks having other hues as may be
necessary. In this case, the order of viscosities of inks having other
hues at 70° C. is the same as in the explanation of the ink-jet
recording method.

[0131] In the present invention, it is preferred that the viscosity of
each in the ink set is 10 mPas or less at 25° C. and 100 mPas or
more at 70° C.

[0132] The ink set produced as described above can be favorably used for
the above-described ink-jet recording method of the present invention.

[0133] The present invention is contemplated for providing an ink jet
recording method capable of preventing dot bleeding especially at the
overlap portion between the first-order color and the second-order color
in formation of the multi-order color image that uses a high-temperature
gelling ink, and capable of achieving high-quality printing with a high
density. In addition, the present invention is contemplated for providing
an ink set capable of achieving high-quality printing with reduced
bleeding and a high density using the above-described ink-jet recording
method.

[0134] According to the ink jet recording method of the present invention,
by adjusting a high-temperature gelling ability of the ink having each
color hue in accordance with the ejecting order of the multi-order color
image printing, it is possible to prevent dot bleeding at the overlap
portion between the first-order color and the second-order color and
further to form a more favorable quality image with a high density at a
high speed.

[0135] The ink set of the present invention can be used for the formation
of a multi-order color image in accordance with the above-described ink
jet recording method whereby dot bleeding can be prevented and
high-quality image with a high density can be formed at a high speed.

[0136] The present invention will be described in more detail based on the
following examples, but the invention is not intended to be limited
thereto. In the following examples, the terms "part(s)" and "%" are
values by mass, unless otherwise specified.

[0137] In addition, the weight average molecular weight was measured by
gel permeation chromatography (GPC). HLC-8220 GPC (trade name,
manufactured by TOSOH CORPORATION) was used for the GPC, and TSK GEL
Super HZM-H, TSK GEL Super HZ4000, and TSK GEL Super HZ2000 (trade names,
all manufactured by TOSOH CORPORATION) were used as the columns and were
connected in a series of three. THF (tetrahydrofuran) was used as the
eluent solution. For the conditions, the sample concentration was 0.35%
by mass, the flow rate was 0.35 mL/min, the amount of sample injection
was 10 the measurement temperature was 40° C., and an RI detector
was used. A calibration curve was prepared from 8 samples of the
"standard sample TSK standard, polystyrene": "F-40", "F-20", "F-4",
"F-1", "A-5000", "A-2500", "A-1000" and "n-propylbenzene" (trade names,
manufactured by TOSOH CORPORATION).

EXAMPLES

Example 1

----Synthesis of Water-Insoluble Polymer Dispersant P-1----

[0138] 88 g of methyl ethyl ketone was added to a 1000-ml three-necked
flask equipped with a stirrer and a condenser tube, and was heated to
72° C. under a nitrogen atmosphere. To this, a solution of 0.85 g
of dimethyl 2,2'-azobisisobutyrate, 60 g of benzyl methacrylate, 10 g of
methacrylic acid and 30 g of methyl methacrylate dissolved in 50 g of
methyl ethyl ketone was added dropwise over a period of 3 hours. After
the addition was completed, the mixture was reacted for additional one
hour, and then a solution of 0.42 g of dimethyl 2,2'-azobisisobutyrate
dissolved in 2 g of methyl ethyl ketone was added. The temperature was
elevated to 78° C., and the mixture was heated for 4 hours. The
obtained reaction solution was precipitated two times in large excess of
hexane, and the precipitated resin was dried to obtain 96 g of a
water-insoluble polymer dispersant P-1.

[0139] The composition of the obtained resin was confirmed by 1H-NMR,
and the weight average molecular weight (Mw) determined by GPC was
44,600. The acid value was determined by the method described in JIS
Standards (JIS K0070: 1992), and the value was 65.2 mgKOH/g.

[0140] Carbon black and other components were mixed according to the
following composition and dispersed with a beads mill using 0.1 mm φ
zirconia beads for 3 to 6 hours. From the resulting dispersion, the
methyl ethyl ketone was removed at 55° C. under reduced pressure,
and further, a part of the water was removed, whereby a dispersion of
resin-coated carbon black particles having a carbon black concentration
of 10.0% by mass was prepared.

[0142] The resulting dispersion of resin-coated carbon black particles
(pigment dispersion) was measured for its volume-average particle size by
a dynamic light scattering method using a particle size distribution
measuring instrument NANOTRAC UPA-EX 150 (trade name, manufactured by
NIKKISO Co., Ltd.). In this measurement, 10 mL of ion-exchanged water was
added to 30 μL, of the resin-coated carbon black particle dispersion
to prepare a measurement sample, and the thus prepared sample was then
measured at a controlled temperature of 25° C. The obtained
particle size was 98 nm.

[0144] From the obtained dispersion, methyl ethyl ketone was removed at
55° C. under reduced pressure, and further water was partially
removed. Then, the resultant was subjected to centrifugal treatment using
a 50 mL centrifuging tube by means of a HIGH-SPEED CENTRIFUGAL COOLER
7550 (trade name, manufactured by Kubota Corporation) at 8000 rpm for 30
minutes, thereby collecting the supernatant excluding the precipitate.
Thereafter, the pigment concentration was determined from the absorbance
spectrum, thereby obtaining a dispersion C of cyan coloring particles
that is a dispersion of resin-coated pigment particles (pigment coated
with a polymer dispersant) and has a pigment concentration of 10.2% by
mass.

[0145] Dispersion Y of yellow coloring particles and dispersion M of
magenta coloring particles were prepared in the same manner as the
above-described dispersion C of cyan coloring particles except that
pigment blue 15:3 in the dispersion C of cyan coloring particles was
changed to pigment yellow 74 (IRGALITE YELLOW GS, manufactured by Ciba
Japan) and pigment red 122 (CROMOPHTAL JET MAGENTA, manufactured by Ciba
Specialty Chemicals Inc.), respectively. The pigment densities of the
dispersion Y of yellow coloring particles and the dispersion M of magenta
coloring particles were 10.3% by mass and 10.1% by mass, respectively.

(Measurement of Particle Diameters)

[0146] The particle diameters of the thus-obtained dispersion Y of yellow
coloring particles, dispersion M of magenta coloring particles and
dispersion C of cyan coloring particles were measured in the same manner
as the dispersion K of carbon black particles. Results of measurement
were as follows. Y: 115 nm, M: 105 nm, C: 97 nm

----Preparation of Black Inks K1 and K2----

[0147] Then, the resulting dispersion of resin-coated carbon black
particles was used to prepare an aqueous ink with the following
composition. A plastic disposable syringe was filled with this aqueous
ink, and then filtrated using a PVDF 5 μm filter (Millex-SV (trade
name), manufactured by MILLIpore Corporate, diameter: 25 mm). Thus, a
black ink (ink-jet ink composition) K1 which gels at high temperature was
obtained. The pH of the aqueous ink at 25° C. was 8.7.

----Preparation of Color Inks Y1, Y2, M1, M2, C1 and C2----Yellow inks Y1
and Y2, magenta inks M1 and M2 and cyan inks C1 and C2 were prepared in
the same manner as the Black ink K1 except that the composition of each
inks was changed to the ink composition (mass part) in Table 1. The
viscosity at 25° C. and 70° C., and pH at 25° C. of
each of the obtained aqueous ink-jet inks were measured. The results are
shown in Table 1. The value of viscosity was defined as an average of the
values obtained by measuring a viscosity five times every 100 seconds
after the test sample has been adjusted to a predetermined temperature
shown in Table 1 using a temperature-valuable type rotational viscometer
Physica MCR301 (trade name, manufactured by Anton Paar GmbH). As the
measuring conditions, shear rate of 10 (1/s) and rate of temperature rise
of 5° C./5 seconds were used. The composition, viscosity and pH of
black inks K1 and K2 were also shown in Table 1.

[0151] An apparatus was prepared in which four piezoelectric heads 1 to 4
(each 384 nozzles) were disposed side by side, and a heater capable of
heating a printing paper was set. In the apparatus, the heads were
arranged so that ejecting was carried out in order of from head 1 to head
4. Each head was filled with black, cyan, magenta or yellow inks
according to Table 2 to prepare an ink set. Further, a recording voltage
was modulated so that the ink droplet size fell in the range of from 7 to
8 pl. A normal paper NPi-55 (trade name, manufactured by Nippon Paper
Industries Co., Ltd., basis weight: 55 g/m2) was set on a heater,
and then heated so that the temperature on the plane of paper became
70° C. After that, by ejecting inks of desired color from
piezo-type recording heads 1 to 4, 1 cm×1 cm-size five-step images
having halftone dot percentage of 100%, 80%, 60%, 40%, and 20% (which
were constituted of the second-order color and the third-order color)
were printed in a single pass mode. The temperature on the plane of paper
was measured using a radiation thermometer (trade name: IR-66B,
manufactured by MK Scientific, Inc.). In addition, the printing condition
is as follows.

[0155] A dot portion of the printed halftone dot image having a halftone
dot percentage of 40% was observed at a magnification of 20 times with a
microscope. Dot bleeding of the second-order color (composite gray with
respect to Example 7 and Comparative Example 4) was evaluated on the
basis of the following criterion. The obtained results are shown in 3.

(Criterion of Bleeding Evaluation)

[0156] A: Dots of different colors from each other are properly separated
at the overlap portion between these dots. B: Bleeding is seen in a part
of dot edge at the overlap portion between dots having different colors
from each other. C: Bleeding is seen in the dot of the later-printed
color at the overlap portion between dots having different colors from
each other.

[0157] As is apparent from Table 3, bleeding was observed at the dot of
later-printed color in Comparative Examples 1 to 4. In contrast,
good-quality printed matters were obtained without any dot bleeding in
Examples 1 to 7. Therefore, it is understood that in the case of using
the ink set in which high-temperature gelling inks having different
colors from each other are used in combination, images improved in terms
of bleeding at the color-overlapped portion can be obtained by setting
the ejecting order so as to depend on the viscosity of the ink at
70° C.

Example II

----Full Color Image Recording----

[0158] Full color image recording was carried out by using ink sets 7 and
11 prepared in Example I in accordance with both the ejecting order and
the recording condition used in Example I. Then, evaluation of
comprehensive image quality was conducted. In addition, images were
printed after selecting three kinds of each of portrait and scenic image.
A sensory evaluation was conducted laying the images having been obtained
by using the ink sets 7 and 11 side by side. As a result, in each of the
images, recording that was carried out by using the ink set 7 (Examples
of the present invention) was able to achieve better-picture quality than
that using the ink set 11 (Comparative Examples). Specifically, the
images obtained by Examples of the present invention were better images
having higher sharpness and less feeling of roughness than the images
obtained by Comparative Examples. Moreover, for the images obtained by
Examples of the present invention, bleeding was also prevented in
multi-order colors such as a second-order color or a third-order color.

[0159] Having described our invention as related to the present
embodiments, it is our intention that the present invention not be
limited by any of the details of the description, unless otherwise
specified, but rather be construed broadly within its spirit and scope as
set out in the accompanying claims.